The present invention relates to frame synchronization and, in particular, a frame synchronization circuit for digital pulse code modulation (PCM) communication equipment.
In digital transmission systems, analog signals such as voice signals, are sampled and converted into a binary code word. The respective bits of code word are then serially transmitted to a receiver wherein the binary digits are recovered, and the code words reconstructed and decoded. The digital code words are then reconverted into an analog signal.
Digital transmission techniques facilitate the use of time division multiplexing to provide for transmission of data from many data channels over a single digital line. The digitized signals from the respective channels are combined serially in a periodic fashion. A typical example of a time division multiplexed PCM transmission system is that of the Bell Telephone system. A so-called "channel bank" receives a plurality of voice channels. The "channel bank" samples each channel in sequence and generates a digital code or byte representative of the sample on the digital line during relative bit time slots corresponding to that channel. Thus, the digital pulse stream comprises digitized signals from a plurality of channels serially combined in a periodic fashion. The periodic signal structure is often referred to as the line format, and one basic period of the signal is generally referred to as a frame. In other words, a signal frame comprises one digitized sample code word from each of the multiplexed channels. In typical applications, the channel bank encodes the samples into 8-bit binary code words for transmission over a 1.544 mega bits/second repeater line.
In demultiplexing the signal at the receiver, it is necessary to identify the beginning of the frame. A framing synchronization system is necessary to identify the various signal components and to bring the receiver into phase with the respective line formats so that the data bits can be properly assembled for decoding and demultiplexing. To identify the frame structure, an identifiable pulse sequence associated with the periodic format of the frame or some multiple of the frame format is inserted into the digital bit stream. The identifiable pulse sequence, hereinafter referred to as the framing characteristics or framing pattern, in effect, defines the frame length. Framing techniques typically utilize synchronization (sync) bits inserted in a predetermined bit time slot (relative bit position) within the frame, following a predetermined pattern. For example, the so-called T1 frame format consists of twenty-four 8-bit sample code words plus an additional sync bit.
The sync bit value pattern is predetermined in accordance with the type of interface equipment and method of encoding supervision data. One common T-carrier sync bit pattern utilizes only the sync bit in every second frame, and alternates the value (between 1 and 0) of such sync bits. The bits of the intervening frames are utilized for common channel signaling, channel bank alarms or for a multi-frame synchronization sequence for more complex encoding of supervision and alarm data. Framing synchronization is accomplished utilizing this pattern by recognizing the 1,0,1,0,1,0 . . . pattern of the alternate frame sync bits. The framing interval is thus twice the frame length and the pairs of frames are, in effect, treated as a single superframe.
Another sync pattern commonly utilized on T1 lines repeats every twelve frames in the following sequence: 1,0,0,0,1,1,0,1,1,1,0,0. In addition, the sixth and twelfth frames are designated as containing "A-sync" and "B-sync" pulses. The A-sync and B-sync pulses are utilized in decoding supervision.
It is desirable that a framing synchronization system be relatively insensitive to line errors in the data stream and that the framing circuit be capable of rapidly establishing the framing sync bit or re-establishing identification of the sync bit after inadvertent loss of frame. Prior art framing systems have typically examined each relative bit time slot in a sequence manner. The framing pattern is verified by comparing the incoming data bits in a given bit position (time slot) over a number of frames with framing digits generated internal to the receiver. If the sync pattern of the time slot under examination is not the expected pulse sequence, the next time slot is searched. This searching scheme continues until a time slot is found containing the sync pattern. It must be appreciated, however, that such sequence framing synchronization techniques necessarily entail reframe times spanning a number of frames. Such a serial technique, in effect, involves searching through all possible bit time slots in order to locate the correct framing sequence.
Inadvertent loss of frame synchronization in the prior art is usually caused by line errors which temporarily alter the values of the sync bits and cause the receiver to declare itself out of frame, and requiring the framing circuit to re-establish frame synchronization. Such an unnecessary reframing (often referred to as a misframe) entails a loss of information during the reframing period.
For a more detailed description of digital transmission and frame synchronization techniques, reference is made to "Transmission Systems for Communications", Bell Telephone Laboratories, Western Electric Company, Inc., Technical Publications, Winston-Salem, North Carolina, 1970.
For further description of synchronization circuits and framing circuits, reference is made to the following U.S. Pat. Nos.: 3,581,010 (Kobayashi, 1971); 3,594,502 (Clark, 1971); 3,649,758 (Clark, 1972); 3,652,799 (Thomas, 1972); 3,662,114 (Clark, 1972); 3,678,200 (Clark, 1972); 3,699,261 (Tomozawa, 1972); 3,705,315 (Clark, 1972); 3,735,045 (Clark, 1973); 3,742,139 (Boehly et al, 1973); U.S. 3,754,102 (Clark, 1973); 3,770,897 (Haussmann et al, 1973); 3,792,201 (Osborne, 1974); 3,798,378 (Epstein, 1974); 3,854,011 (Mallory et al, 1974); 3,867,579 (Colton et al, 1975); 3,903,371 (Colton et al, 1975); 3,920,900 (Fineman, 1975); 3,985,967 (Colton et al, 1976); 3,988,674 (Sciulli, 1976); 4,002,845 (Kaul et al, 1977); 4,004,100 (Takimoto, 1977); 4,010,325 (Kline, 1977); 4,016,368 (Apple, Jr., 1977); 4,060,797 (Maxwell et al, 1977); and 4,121,057 (Luder, 1978).